Dichromatic gelatin-based (DCG) volume-phase holographic optical elements (HOEs) are used in a variety of applications and industries including spectroscopy, displays and fiber-optic communication and laser delivery systems. The optical performance advantages of these elements is well known but, due to the humidity sensitivity of the gelatin recording media, they are difficult to harden against the environment.
According to current techniques, the HOE is supported in a laminated sandwich, with the gelatin 102 being disposed between glass plates 104. However, the edges 106 of the gelatin remain exposed. Many sealing methods have been employed, most of which rely on some type of adhesive, typically epoxy. The most successful approach to date is to chemically etch the gelatin back from the edge of the glass and then laminate the glass cover. This creates an epoxy barrier 110 to the environment, as shown in FIG. 1.
Environmental specifications, particularly those used in the telecommunications industry, continue to become more stringent, however. So far, one of the most stringent specifications is the GR-1221-CORE Bellcore spec. One of the tests in this specification is known as “damp heat,” which requires that the optical element to suffer no ill effects when exposed to 85° C. and 85 percent humidity for at least 500 hours. Extensive testing has not yielded any adhesive-based sealing method that adequately passes this test.